1. Field of the Disclosure
This disclosure relates to a process for free radical and for living or controlled polymerization of alkene monomers (e.g., fluorine substituted alkene monomers), particularly the use of hypervalent iodide (HVI) radical initiators in the living or controlled polymerization of alkene monomers and for the functionalization of (organic) substrates with the CF3 or RF groups.
2. Discussion of the Background Art
Conventional chain polymerization of vinyl monomers usually consists of three main elemental reaction steps: initiation, propagation, and termination. Initiation stage involves creation of an active center from an initiator. Propagation involves growth of the polymer chain by sequential addition of monomer to the active center. Termination (including irreversible chain transfer) refers to termination of the growth of the polymer chain. Owing to the presence of termination and poorly controlled transfer reactions, conventional chain polymerization typically yields a poorly controlled polymer in terms of molecular weight and polydispersity which control the polymer properties. Moreover, conventional chain polymerization processes mostly result in polymers with simple architectures such as linear homopolymer and linear random copolymer.
Living polymerization is characterized by the absence of any kinds of termination or side reactions which might break propagation reactions. The most important feature of living polymerization is that one may control the polymerization process to design the molecular structural parameters of the polymer. Additional polymerization systems where the termination reactions are, while still present, negligible compared to propagation reaction are known in the art. As structural control can generally still be well achieved with such processes, they are thus often termed “living” or controlled polymerization.
In living or controlled polymerization, as only initiation and propagation mainly contribute to the formation of polymer, molecular weight can be predetermined by means of the ratio of consumed monomer to the concentration of the initiator used and will increase linearly with conversion. The ratio of weight average molecular weight to number average molecular weight, i.e., molecular weight distribution (Mw/Mn), may accordingly be as low as 1.0, and the polymers have well defined chain ends. Moreover, polymers with specifically desired structures and architectures can be purposely produced. In terms of topology, such structures and architectures may include linear, star, comb, hyperbranched, dendritic, cyclic, network, and the like. In terms of sequence/composition distribution, such structures and architectures may include homopolymer, random copolymer, block copolymer, graft copolymer, gradient copolymer, tapered copolymer, periodic copolymer, alternating copolymer, and the like. In terms of functionalization, such structures and architectures may include telechlics, macromonomer, labeled polymer, and the like.
Living polymerization processes have been successfully used to produce numerous polymeric materials which have been found to be useful in many applications. However, many living polymerization processes have not found wide acceptance in industrial commercialization, mainly due to high cost to industrially implement these processes. Thus, searching for practical living polymerization processes is a challenge in the field of polymer chemistry and materials.
Additionally, as (co)polymers of main chain fluorinated monomers (e.g., vinylidene fluoride (VDF), hexafluoropropene, tetrafluoroethylene, trifluorochloroethylene, and the like) are industrially significant, the study of their controlled radical polymerization and the synthesis of complex polymer architectures thereby derived, would be desirable. However, such polymerizations are challenging on laboratory scale, as bpVDF=−83° C. Thus, telo/polymerizations are carried out at T>80-150° C. and require high-pressure metal reactors.
Kinetic studies of VDF polymerizations involve many one-data-point experiments as direct sampling is difficult. This is very time-consuming and expensive due to the typical lab unavailability of a large number of costly metal reactors, which moreover require tens of grams of monomer. The development of methods that would allow small scale (e.g., a few grams) VDF polymerizations at ambient temperature in inexpensive, low pressure glass tubes, would be highly desirable, since the methods could easily be adapted for fast screening of a wide range of polymerization and of reaction conditions, and could also take advantage of photochemistry. The development of such methods would also be useful on a large scale, for example, in an industrial setting. Conventional initiating systems such as peroxides or redox systems do not initiate the polymerization of VDF at ambient or room temperature.
Driven by the unique properties imparted by the —CF3 moiety onto chemical structures ranging anywhere from synthetic drugs to polymers and nanostructures, trifluoromethylation (TFM) has recently emerged as a very valuable technique towards improving and expanding molecular properties and functions.
As such, while the vast majority of TFM reactions involve nucleophilic (“CF3−” e.g., Me3Si—CF3), electrophilic (“CF3+” e.g. chalcogen salts [CF3—YAr2]OTf, Y═O, S, Se, Te, or cyclic iodanes such as 1-trifluoromethyl-1,2-benziodoxole, CF3—I(-Ph-OCO—) as well as organometallic (e.g., “CF3—Cu”, or Pd, Ni) protocols for arene or carbonyl TFM functionalization, very recently, radical (CF3.) aryl (CF3SO2Na/tBuOOH), enantioselective carbonyl (CF3I/RuCl2(PPH3)3) as well as photomediated aryl (Ru(phen)3Cl2/CF3SO2Cl) and carbonyl (CF3—I) α-TFMs have emerged as a much more/very convenient/inexpensive/very powerful strategies for the rapid synthesis of TFM-lated libraries with wide structural diversity.
Conversely, fluorinated (co)polymers derived from radical reactions are a fundamental class of specialty materials endowed with a wide range of high-end applications which require their precise synthesis. However, while modern state-of-the-art controlled radical polymerizations (CRP) methods (atom transfer, nitroxide or reversible addition-fragmentation) have undergone remarkable developments for conventional monomers such as (meth)acrylates or styrene, they remain ineffective for the highly reactive, gaseous main chain fluorinated alkene monomers (FMs: vinylidene fluoride (VDF), hexafluoropropene (HFP), tetrafluoroethylene, etc).
Thus, due to the current lack of suitable CRP chemistry, the synthesis, characterization and fundamental understanding of the self-assembly, properties and applications of well-defined FM complex macromolecular architectures (blocks, graft, hyperbranched, stars, etc.) still lag significantly behind those associated with the corresponding materials derived from conventional alkenes (styrene, acrylates, dienes, etc.).
To date, industrial FM-CRP is still accomplished with the oldest of CRP methods, theiodine degenerative transfer (IDT: Pn.+Pm—IPn—I+Pm.), which evolved from high temperature (100-250° C.) free radical VDF telomerizations with polyhalides, and especially (per)fluorinated iodine (RF—I) chain transfer (CT) agents, including CF3—I or I—(CF2)n—I.
However, while the RF—I derived electrophilic RF. radicals add readily to nonfluorinated alkenes at room temperature (rt) under metal catalysis, and many metal complexes activate typical alkyl halide (R-X) ATRP initiators,only very low VDF oligomers are obtained, even at T>100° C. from transition metal salts and polyhalides. Moreover, although VDF polymerization can proceed at room temperature (rt), the metal mediated radical initiation of such electrophilic FMs directly from halides and thus metal-mediated FM-CRP at T<100° C., including around rt, is not available. Consequently, conventional FM-IDT always demands a free radical initiator (e.g. tbutyl peroxide).
As such, the development of FM-CRP, the synthesis of elaborate FM polymer architectures, and the mapping the resulting fluoromaterials genome remains a worthy endeavor. Conversely, such polymerizations are very challenging especially in an academic laboratory scale/setting, as all FMs are gases (bpVDF=−83° C.) and typical telo/polymerizations are carried out at T>100-200° C., in expensive, high-pressure metal reactors.
Moreover, in additional contrast with acrylates- or styrene-CRP, VDF-IDT generates two halide chain ends, Pn—CH2—CF2—I and Pm—CF2—CH2—I with vastly different reactivity, and, while acrylate or styrene kinetics can effortlessly be sampled even on a 1 g scale, FM polymerizations involve many time-consuming one-data-point reactions using at least tens of grams of monomer.
Thus, development of mild temperature protocols for low pressure, small-scale polymerizations in inexpensive glass tubes, would be very appropriate for fast catalyst and reaction condition screening and also amenable to photochemistry. As such, while VDF high power UV telomerizations exist, until recently, there were no reports on VDF polymerizations under regular visible light.
While CH3. is also available from, for example, the decomposition of TBPO, the generation of CF3. from CF3—I is expensive and impractical (bpCF3I=−22.5° C.). In fact, except for Mn2(CO)10 experiments above, very few other CF3. precursors have ever been evaluated in the initiation of FMs, where such radicals were generated either by high temperature thermolysis or under strong UV irradiation from commercially available but inconvenient and expensive CF3—Br and CF3—I, or from commercially unavailable CF3—SO2—SR, CF3—S—(C═S)—OR, explosive CF3—C(O)O—O(O)C—CF3, toxic Hg(CF3)2, Cd(CF3)2, Te(CF3)2, or from even more exotic and expensive substrates such as CF3-decorated octafluoro[2.2]paracyclophane or persistent perfluoro-3-ethyl-2,4-dimethyl-3-pentyl radicals. Thus, availability of a clean, safe, nongaseous, commercially available and inexpensive source of CF3. radicals would be highly desirable for TFM radical reactions involving either polymerizations or arene functionalization.
Interestingly, although known for over a century, hypervalent iodine(III,V) (HVI) derivatives (λ3-and λ5-iodanes) have recently undergone a resurgence in organic chemistry. Consequently, they have also become inexpensively commercially available, as illustrated especially by acyloxyiodobenzenes such as (CX3COO)2IIIIPh, (X=H, I-DAB, X=F, I—FDAB) and (CH3COO)3IV(-Ph-CO—O—) (Dess-Martin cyclic periodinane, DMP,), or to a lesser extent, by diaryliodonium salts (Ar2I+Y−, Y=PF6, OTf, etc.
While the overwhelming majority of such HVI carboxylates applications are oxidations, examples of radical processes are also emerging. Thus, alkyl radicals obtained thermally or under Hg—UV from the decarboxylation of HVIs derived in-situ by ligand exchange of IDAB and IFAB with carboxylic acids, add to alkenes or alkylate heteroaromatic bases. Alternatively, in the additional presence of I2, HVIs mediate the hypoiodite reaction of R—Y—H such as alcohols, carboxylic acids, and amines to generate transient R—Y—I, which upon UV-VIS irradiation provide the corresponding R—Y. radicals (Y═O, COO, NR).
However, while diaryliodonium salts are known cationic polymerizations photoinitiators and photoacid generators in photolithography, the potential use IDAB and IFAB as radical polymerization initiators, remains largely ignored and, to the best of our knowledge, there are no reports on the use of IDAB and IFAB as initiators for the radical polymerization of fluorinated monomers, on the use of IFAB in trifluoromethylation reactions, and on the photolysis of DMPI and its radical reactions.
It would be desirable to provide a method for living polymerization of alkene and fluoroalkene monomers which provides a high level of macromolecular control over the polymerization process and which leads to uniform and more controllable polymeric products. It would be especially desirable to provide such a living polymerization process with existing facility, and which enables the use of a wide variety of readily available starting materials. It would be further desirable to provide a method that would allow small scale (e.g., a few grams) VDF polymerizations at ambient temperature in inexpensive, low pressure glass tubes, and also large scale VDF polymerizations, for example, in industrial settings. The glass tubes as well as metal reactors could also take advantage of photochemistry.
The present disclosure also provides many additional advantages, which shall become apparent as described below.